Solar cells and solar modules convert sunlight into electricity. These electronic devices have been traditionally fabricated using silicon (Si) as a light-absorbing, semiconducting material in a relatively expensive production process. To make solar cells more economically viable, solar cell device architectures have been developed that can inexpensively make use of thin-film, light-absorbing semiconductor materials such as copper-indium-gallium-selenide (CIGS) and the resulting devices are often referred to as CIGS solar cells.
A central challenge in cost-effectively constructing a large-area CIGS-based solar cell or module involves reducing processing costs and material costs. In known versions of CIGS solar cells, the transparent electrode layer and many other layers are deposited by a vacuum-based process over a rigid glass substrate. Typical deposition techniques include co-evaporation, sputtering, chemical vapor deposition, or the like. The nature of vacuum deposition processes requires equipment that is generally low throughput and expensive. Vacuum deposition processes are also typically carried out at high temperatures and for extended times. Traditional sputtering or co-evaporation techniques are limited to line-of-sight and limited-area sources, tending to result in poor surface coverage and non-uniform three-dimensional distribution of the elements.
High-efficiency thin-film solar cells based on polycrystalline CIGS (copper indium gallium di-selenide, but not excluding any other of the IB, IIIA, VIA elements like e.g. aluminum, and sulfur) are typically made with a transparent conductive oxide (TCO) deposited on top of a stack containing the CIGSe film, where depending on interconnect scheme some require additional conductive patterns (traces, fingers, grids, lines, bus bars, etc.) to collect the current with minimal electrical-resistive and optical-shadowing losses. Lowering the cost of the deposition of these transparent conductive layers and conductive patterns is required to minimize the overall cost of the solar panels.
One of the most common techniques used to roll-to-roll deposit transparent conductive electrodes (TCE) is sputter deposition of transparent conductive oxides (TCO). Unfortunately, for the film thickness and high vacuum required, sputter deposition is a slow process with an undesired low throughput/capex ratio. In addition, material yield is low due to deposition of material onto the chamber walls. Furthermore, temperature control during sputter deposition can limit the throughput even further, especially when damage of underlying temperature-sensitive layers, like e.g. the CIGSe/CdS stack, needs to be prevented. Finally, controlling the large-area uniformity of both the conductivity and transparency of a sputter-deposited TCO is challenging.
In an attempt to lower manufacturing cost, solution-deposition of transparent conductive layers has been investigated by others. One example uses high-temperature sintering of solution-deposited metal oxide particles and subsequently these particles are sintered to a dense layer. A huge disadvantage of this method is the temperature required to get dense layers. Temperatures required to sinter these layers damage the underlying films as used in thin-film PV.
Other examples use individually grown carbon nanotubes or metallic nanowires that are solution-deposited and where the layer is mechanically stabilized by organic additives allowing processing at temperatures that prevent damage to the underlying layers. One big disadvantage of this approach is the limited lateral electrical conductivity that can be accomplished without loosing too much optical transparency. Additionally, since the amount of organics required to mechanically stabilize these layers is considerable, a bi-continuous percolating conductive network inside an electrically-insulating network is created. The two-phase nature of this approach limits the contact area that can be achieved at both interfaces when sandwiched between other (semi)conductive layers. This limited contact area affects the contact resistance in a negative way. Additionally, a possible mismatch in the coefficients of thermal expansion of both the conductive and insulating materials might impact the temperature dependence and reliability of the overall conductivity of the bi-continuous network in a negative way.
A third approach mixes organic nanotubes with doped conjugated polymers to increase the lateral conductivity compared to nanotubes only. Typically, the chemical stability of organic materials is photosensitive and the conductivity is smaller than of inorganic materials.
Summarizing, the major challenges to uniformly deposit a highly-conductive, highly-transparent (for AM1.0 or AM1.5), chemically-stable, reliable thin layer onto a large area without use of vacuum and high temperature.
Due to the aforementioned issues, improved techniques may be used for reducing processing costs and material costs. Improvements may be made to increase the throughput of existing manufacturing processes and decrease the cost associated with CIGS based solar devices. The decreased cost and increased production throughput should increase market penetration and commercial adoption of such products.